In injection molding machines the core pin is first inserted into the mold cavity of an injection mold where the surfaces of the cavity are spaced from the surfaces of the core pin by a distance equal to the desired thickness of plastic coating which is to be applied to the core pin. The plastic material is then injected into the mold cavity and the core pin thus coated with the material.
The mold is then opened and the core pin is transferred to a blowing mold where the cavity has the shape of the product which is to be blown. Blowing fluid under pressure is then discharged from the core pin against the plastic that surrounds the core pin and the plastic is blown outwardly away from the core pin and into contact with the sides of the mold. After a short cooling time, the blowing mold is opened and the core pin is moved to a stripping station where the blown article is stripped from the core pin.
In extrusion molding machines, a plastic tube is extruded downwardly into an open blowing mold and a core pin extends from the extruder downwardly along the axis of the tube. When the tube has reached a length greater than the length of the blowing mold, the mold closes and pinches the bottom of the tube closed while clamping the upper end of the tube tightly around the core pin. The tube is then blown to the shape of the mold cavity by blowing fluid discharged from the core pin and the mold is opened to permit the extrusion of another length of tubing. The blown molding is cut off from the tube and the mold closes on the new length of tube to repeat the blowing step.
In both types of machines, the molding must remain in the blowing mold until the plastic cools sufficiently to hold its shape. The blowing mold is cooled by circulation of water or other cooling fluid through cooling passages in the wall of the mold.
The pressure of air or other fluid used to blow the plastic must be limited in pressure. If introduced into the blowing mold at too high pressure, the blowing fluid will blow out the plastic wall at the weakest place or location of greatest stress, and the temperature of the plastic must be controlled so that the plastic is hot enough to blow at the pressure that can be safely used.
By covering the core rod with a balloon and coating the outside of the balloon with the plastic to be blown, a number of new results are obtained. One is that higher pressure can be used to blow the molding. The balloon prevents the blowing fluid from having contact with the plastic and there is no danger of a blowout of the plastic. Because higher pressure can be used for blowing, plastic can be blown at lower temperature and after it has been cooled to the temperature range at which it begins to crystallize. This permits orientation of the molecules and by having a balloon that is deformed biaxially, the plastic of the molding is stretched both circumferentially and axially (or in other directions at right angles to one another) so that the molding has biaxial orientation with resulting greater strength, and in the case of some plastics, clear transparency.
Another new result is obtained if the balloon is made with different wall thickness at different sections of its extent. There is then a sequential expansion of the different sections of the balloon so that corresponding sections of the parison are blown at different times. For example, the upper or lower portion of the molding might be blown ahead of another portion.
Blowing fluid, such as liquid, at temperatures substantially lower than the plastic can be used and circulated through the balloon during the blowing operation and immediately thereafter so that the molding is cooled from both the inside and outside to obtain substantially faster cooling and a shorter cycle for the machine, and increased production.
The invention is suitable for use on either injection blow molding machines or extrusion blow molding machines.
Other objects, features, and advantages of the invention will appear or be pointed out as the description proceeds.